1. Field of the Invention
The present invention relates to a drive controller that performs micro-step drive of a stepping motor.
2. Description of the Related Art
Conventionally, a stepping motor has been widely used to drive a focus lens in an automatic focus control (AF control) by a TV-AF method in an image pickup apparatus. The AF control by the TV-AF method detects the position of the focus lens at which an AF evaluation value is maximized, by performing a repetitive operation of micro movement (a wobbling operation) in both telephoto side and wide-angle side directions of the focus lens. A micro-step drive method is widely adopted as a method of driving the stepping motor for the AF control by the TV-AF method. The micro-step drive method employs an excitation current having a sine waveform to achieve drive with a step (micro step) finer than one step (the minimum rotation angle) of the stepping motor.
In the micro-step drive method, however, the rotation angle of the micro step fluctuates with the phase (step phase) of the excitation current. This is because deviation (cogging) of the magnetic flux distribution of a permanent magnet that is a rotor of the stepping motor causes pulsation of a magnetic attracting force. Fluctuation in the rotation angle due to the cogging at each step phase is a factor causing fluctuation in the moving amplitude of the focus lens in the wobbling operation.
When a wobbling width (the number of micro steps) is determined based on the step phase having a small rotation angle, the amount of movement of the focus lens exceeds the depth of field in some cases in the step phase having a large rotation angle during the wobbling operation, which results in focusing fluctuation. On the other hand, when the wobbling width is determined based on the step phase having a large rotation angle, kinetic energy required for the micro movement of the focus lens cannot be obtained in the step phase having a small rotation angle during the wobbling operation, which potentially causes what is called a step-out.
Japanese Patent Laid-open No. 2011-135700 discloses a drive control method for keeping substantially constant the drive amount of the wobbling operation by controlling the rotation angle to be small when driving through a first phase having a small rotation angle of the stepping motor before and after a second phase having a large rotation angle.
The drive control method disclosed in Japanese Patent Laid-open No. 2011-135700 is effective in the wobbling operation in which the rotation angle is allowed to be large to a certain extent. The effect due to the cogging is, however, unavoidable when a markedly small amount of the wobbling operation is performed to drive only in a region having a large cogging. It is also impossible to controllably change the wobbling amplitude to an appropriate amplitude